flat spot

Hey everyone I'm new to the site and new to being a mommy. I just wanted to know if anyone has ever experienced a flat spot on their babies head. My 4 month old has a flat spot on the back of her head. The doctor doesn't seem concerned and just said it's from being on her back to much and to do more tummy time. Although I am guilty of not a lot of tummy time my daughter really isn't on her back that much. At least I don't think so. She is only on her back when she sleeps or is in the car seat otherwise I am holding her or she is in her bouncer. Am I overreacting to this will it clear up on its own? Had anyone else experienced this? Thanks in advance for any help.